Communication device, communication method, and communication program

ABSTRACT

A communication device includes an interleaving unit that determines an interleaving length of transmit data to be transmitted through free-space optical communication, and interleaves the transmit data based on the determined interleaving length, and a shaping unit that shapes the interleaved transmit data so as to make the interleaving length detectable on a receiving side of the free-space optical communication.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.17/373,909, filed Jul. 13, 2021, which is a continuation of U.S.application Ser. No. 16/801,155, filed Feb. 26, 2020 (now U.S. Pat. No.11,063,666), which claims priority to Japanese Patent Application No.2019-149513, filed Aug. 16, 2019, the entire contents of each areincorporated herein by its reference.

BACKGROUND 1. Field

The present disclosure relates to a communication device, acommunication method, and a communication program.

2. Description of the Related Art

Wireless communication using a radio wave as a transmission medium isconventionally well known. However, in recent years, free-space opticalcommunication (also called “optical wireless communication”) using lightas a transmission medium has also begun to attract attention.Conventional techniques are described, for example, in JapaneseLaid-open Patent Publication No. H11-98086.

Communication using an optical fiber as a transmission path is wellknown as the optical communication. Techniques used in the optical fiberare supposed to be also applicable to the free-space opticalcommunication. However, since the optical fiber greatly differs from thefree-space optical communication in transmission path characteristics,simply applying the techniques used in the optical fiber to thefree-space optical communication does not necessarily achieve highcommunication performance (such as high quality and low delay).

Therefore, the present disclosure proposes a communication device, acommunication method, and a communication program capable of achievingthe free-space optical communication at a high performance level.

SUMMARY

It is an object of the present disclosure to at least partially solvethe problems in the conventional technology.

To solve the above problem, a communication device includes: aninterleaving unit configured to determine an interleaving length oftransmit data to be transmitted through free-space opticalcommunication, and interleave the transmit data based on the determinedinterleaving length; and a shaping unit configured to shape theinterleaved transmit data so as to make the interleaving lengthdetectable on a receiving side of the free-space optical communication.

The above and other objects, features, advantages and technical andindustrial significance of this invention will be better understood byreading the following detailed description of presently preferredembodiments of the invention, when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a configuration example of acommunication system according to an embodiment of the presentdisclosure;

FIG. 2 is a diagram illustrating a specific configuration example of thecommunication system;

FIG. 3 is a diagram illustrating a configuration example of a terminaldevice according to the embodiment of the present disclosure;

FIG. 4 is a diagram illustrating a configuration example of a serverdevice according to the embodiment of the present disclosure;

FIG. 5 is a diagram illustrating a configuration example of acommunication device according to the embodiment of the presentdisclosure;

FIG. 6 is a diagram illustrating a configuration example of anothercommunication device according to the embodiment of the presentdisclosure;

FIG. 7 is a diagram illustrating a relation in physical layerconfiguration between the Open Systems Interconnection (OSI) referencemodel and a free-space optical modem;

FIG. 8 is a diagram illustrating an example of a block format of a basicblock;

FIG. 9 is a diagram illustrating another example of the block format ofthe basic block;

FIG. 10 is a diagram illustrating an error correction block format whenan interleaving length has been extended;

FIG. 11 is another diagram illustrating the error correction blockformat when the interleaving length is extended;

FIG. 12 is a diagram for explaining a basic operation of thecommunication system;

FIG. 13 is a diagram for explaining transmit data processing of theembodiment; and

FIG. 14 is a diagram for explaining received data processing of theembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following describes an embodiment of the present disclosure indetail based on the drawings. In the following embodiment, the sameparts are denoted by the same reference numerals, and thus, descriptionthereof will not be repeated.

The present disclosure will be described according to the order of itemslisted below.

1. Introduction

2. Configuration of Communication System

-   -   2-1. Overall Configuration of Communication System    -   2-2. Specific Configuration Example of Communication System    -   2-3. Configuration of Terminal Device    -   2-4. Configuration of Server Device    -   2-5. Configuration of Communication Device (Ground Station)    -   2-6. Configuration of Communication Device (Satellite Station)    -   2-7. Relation in Physical Layer Configuration between OSI        Reference Model and Communication Device

3. Error Correction Block Format

-   -   3-1. Basic Configuration    -   3-2. Extended Configuration

4. Operations of Communication System

-   -   4-1. Basic Operation    -   4-2. Transmit Data Processing    -   4-3. Received Data Processing

5. Modifications

-   -   5-1. Modifications in terms of Processing    -   5-2. Modifications in terms of Device Configuration    -   5-3. Other Modifications

6. Conclusion

1. INTRODUCTION

In recent years, free-space optical communication (also called “opticalwireless communication”) using light as a transmission medium has begunto attract attention. Communication using an optical fiber as atransmission path is well known as the optical communication, andtechniques used in the optical fiber are supposed to be also applicableto the free-space optical communication. However, since the opticalfiber greatly differs from the free-space optical communication intransmission path characteristics, simply applying the techniques usedin the optical fiber to the free-space optical communication does notnecessarily achieve high communication performance (such as high qualityand low delay).

For example, in an optical communication system using the optical fiberas the transmission path, a data error rate increases with increase inspeed. Therefore, the optical communication system using the opticalfiber as the transmission path introduces forward error correction (FEC)to ensure sufficient communication quality. This error correctionmechanism is supposed to be also applicable to the free-space opticalcommunication. However, the error correction method used in the opticalcommunication system using the optical fiber as the transmission path isan error correction method based on an assumption that the data errordistribution is that of random errors caused by deterioration insignal-to-noise ratio (S/N ratio) of received signals. Therefore, manycases are supposed to occur where the correction cannot correct bursterrors caused by atmospheric disturbances that are likely to occur inthe free-space optical communication.

In the case of optimizing a communication method according to thetransmission path characteristics of the free-space opticalcommunication, a concern is also present that communication methods lackcompatibility with one another, and the development cost and theintroduction risk of a communication system increase as a whole.

Therefore, the present embodiment takes the following measures to solvethis problem.

A communication system of the present embodiment is a free-space opticalcommunication system including communication devices capable of thefree-space optical communication. Each of the communication devices is,for example, an optical transmitter/receiver having a forward errorcorrection function.

One of the communication devices serving as a transmission side includesan interleaving unit (for example, an interleaver) configured to becapable of changing an interleaving length, and a frame synchronizationsignal generation circuit. The communication device serving as thetransmission side determines the interleaving length of transmit data tobe transmitted through the free-space optical communication. Theinterleaving unit interleaves the transmit data based on the determinedinterleaving length. The communication device serving as thetransmission side shapes the interleaved transmit data so as to make theinterleaving length detectable on a receiving side of the free-spaceoptical communication. For example, the frame synchronization signalgeneration circuit included in the communication device appends a framesynchronization signal allowing the receiving side to determine theinterleaving length.

Another of the communication devices serving as the receiving sideincludes a deinterleaving unit (for example, a deinterleaver) configuredto be capable of changing the interleaving length, and a framesynchronization signal receiving circuit. The communication devicesserving as the receiving side automatically adjusts the interleavinglength based on the received frame synchronization signal.

The communication devices included in the communication system may havea function of communicating data exclusively for communication betweendevices, in addition to payload data. The communication system may beconfigured to cause the communication devices to negotiate with eachother so as to be capable of performing transmission and reception at aninterleaving length optimal for the transmission path.

This configuration allows the communication system to achieve thefree-space optical communication at a high performance level. Forexample, the communication system of the present embodiment can changethe interleaving length to any length, and therefore, can achieve stablecommunication by changing the interleaving length according tocharacteristics of a transmission space.

The above has described the outline of the present embodiment. Thefollowing describes a communication system 1 of the present embodimentin detail.

2. CONFIGURATION OF COMMUNICATION SYSTEM

The communication system 1 is, for example, a system for transmittingvarious types of data, such as packet data. The data transmitted by thecommunication system 1 may be stream data (for example, broadcastingstream data), such as moving image data or audio data. The broadcastingcan also be regarded as a kind of communication.

The communication system 1 includes a plurality of communication devicescapable of performing the free-space optical communication, and thecommunication devices perform the free-space optical communication withone another. The term “free-space optical communication” refers hereinto wireless communication performed using light such as infrared lightor visible light (for example, an electromagnetic wave having awavelength ranging from that of infrared rays to that of visible rays).The term “free-space optical communication” can also be called “opticalspace communication” or “optical wireless communication”. The light usedfor the free-space optical communication may be laser light orsynchrotron radiation. In the case of using the infrared light as thelight for the free-space optical communication, the light used for thefree-space optical communication may be light having a wavelength in a1500 nm band longer than a wavelength in a 790 nm band used for acompact disc (CD).

The following specifically describes a configuration of thecommunication system 1.

2-1. Overall Configuration of Communication System

FIG. 1 is a diagram illustrating a configuration example of thecommunication system according to the embodiment of the presentdisclosure. The communication system 1 includes a terminal device 10, aserver device 20, a communication device 30, and a communication device40. While the example of FIG. 1 illustrates each of the terminal device10, the server device 20, the communication device 30, and thecommunication device 40 as one device, the communication system 1 mayinclude a plurality of the terminal devices 10, the server devices 20,the communication devices 30, and the communication devices 40.

The devices in FIG. 1 may be regarded as devices in a logical sense. Inother words, some of the devices in FIG. 1 may be implemented as virtualmachines (VMs), containers, or dockers, and may be physicallyimplemented on the same hardware.

In the present embodiment, the concept of the term “communicationdevice” includes not only a portable mobile device (terminal device),such as a mobile terminal, but also a device installed on a structure ora mobile object. The structure and the mobile object themselves may beregarded as the communication devices. The concept of the term“communication device” includes not only a terminal device, but also abase station device and a relay device. The communication device is akind of a processing device and an information processing device. Thecommunication device can also be called “transmitting device” or“receiving device”.

Terminal Device

The terminal device 10 is an information processing device capable ofexchanging data with a communication device, such as the communicationdevice 30, having a free-space optical communication function. Theterminal device 10 may be the communication device itself having thefree-space optical communication function. The terminal device 10 is,for example, a mobile phone, a smart device (smartphone or tablet), apersonal digital assistant (PDA), or a personal computer (PC). Theterminal device 10 may be a device, such as a professional camera havinga communication function, or may be, for example, a motorcycle or anoutside broadcast van provided with communication equipment, such as afield pickup unit (FPU). The terminal device 10 may be amachine-to-machine (M2M) device or an Internet of Things (IoT) device.The terminal device 10 can be regarded as a kind of the communicationdevice. The terminal device 10 may be connected to another device (suchas the communication device 30) through a wired connection or a wirelessconnection.

The terminal device 10 may be a mobile device. The term “mobile device”refers herein to a movable information processing device. In this case,the terminal device 10 may be an information processing device installedon a mobile object, or may be the mobile object itself. For example, theterminal device 10 may be a vehicle, such as an automobile, a bus, atruck, or a two-wheeled motor vehicle, that moves on a road, or may be awireless communication device mounted on the vehicle. The mobile objectmay be a mobile terminal, or may be a mobile object that moves on land(narrow sense expression of “on the ground”), underground, on water, orunderwater. The mobile object may be a mobile object, such as a drone ora helicopter, that moves in the atmosphere, or may be a mobile object,such as an artificial satellite, that moves outside the atmosphere.

The terminal device 10 need not be a device directly used by a person.The terminal device 10 may be a sensor, such as what is called amachine-type communication (MTC) sensor, installed on, for example, amachine in a factory. The terminal device 10 may be a machine-to-machine(M2M) device or an Internet of Things (IoT) device. The terminal device10 may be a device having a relay communication function, as representedby a device-to-device (D2D) communication function and avehicle-to-everything (V2X) communication function. The terminal device10 may be a device called client premises equipment (CPE) used for, forexample, wireless backhaul communication.

The terminal device 10 may be an optical communication device having thefree-space optical communication function. In this case, the terminaldevice 10 may have the optical communication function provided by thecommunication device 30 or the communication device 40 to be describedlater, and serve as the communication device 30 or the communicationdevice 40. In this case, the terminal device 10 can be regarded as thecommunication device 30 or the communication device 40 itself.

Server Device

The server device 20 is an information processing device capable ofexchanging data with a communication device, such as the communicationdevice 40, having the free-space optical communication function. Theserver device 20 may be the communication device itself having thefree-space optical communication function. For example, the serverdevice 20 is a host computer for use as a server that processes requestsfrom client computers (such as the terminal device 10). The serverdevice 20 may be a PC server, a mid-range server, or a mainframe server.The server device 20 can also be regarded as a kind of the communicationdevice. The server device 20 may be connected to another device (such asthe communication device 40) through a wired connection or a wirelessconnection. The server device 20 can also be called, for example, “cloudserver device”, “local server device”, “management device”, or“processing device”.

The server device 20 may be an optical communication device having thefree-space optical communication function. In this case, the serverdevice 20 may have the optical communication function provided by thecommunication device 30 or the communication device 40 to be describedlater, and serve as the communication device 30 or the communicationdevice 40. In this case, the server device 20 can be regarded as thecommunication device 30 or the communication device 40 itself.

The server device 20 can be used, operated, and/or managed by variousentities (subjects). For example, mobile network operators (MNOs),mobile virtual network operators (MVNOs), mobile virtual networkenablers (MVNEs), neutral host network (NHN) operators, enterprises,educational institutions (such as school corporations and school boardsof local governments), managers of real estates (such as buildings andapartments), and private persons can be assumed as the entities.

The subjects to use, operate, and/or manage the server device 20 arenaturally not limited to the above-listed entities. The server device 20may be installed and/or operated by one business operator, or may beinstalled and/or operated by one private person. The subjects to installand/or operate the server device 20 are naturally not limited to thesesubjects. For example, the server device 20 may be installed and/oroperated by a plurality of business operators, or installed and/oroperated by a plurality of private persons. The server device 20 may becommon equipment used by a plurality of business operators or aplurality of private persons. In this case, the equipment may beinstalled and/or operated by a third party different from the users.

The server device 20 provides a predetermined communication service tothe terminal device 10 through an optical communication device, such asthe communication device 40. For example, the server device 20 provides,to the terminal device 10 on which a predetermined application programis installed, through wireless communication, an execution service ofinformation processing (hereinafter called “application processing”)requested by the application program.

The application processing performed by the server device 20 refers to,for example, information processing, such as recognition processing ofan object in an image, at the application layer level that is performedbased on a request from a computer program (such as an application)provided on a mobile device, or performed in cooperation with thecomputer program. For example, the application processing performed bythe server device 20 may be what is called edge processing in edgecomputing. The application processing differs from processing at any ofwhat are called the physical layer level, the data link layer level, thenetwork layer level, the transport layer level, the session layer level,and the presentation layer level in the OSI reference model. However, ifthe application processing includes processing, such as imagerecognition processing, at the application layer level, the applicationprocessing may secondarily include the processing at any of the physicallayer level to the presentation layer level.

Communication Device (Ground Station)

The communication device 30 is an optical wireless communication devicethat uses light to wirelessly communicate with the communication device40 or another communication device 30. The communication device 30 is,for example, an optical communication modem. The communication device 30is not limited to the optical communication modem. For example, thecommunication device 30 may be a device corresponding to a wireless basestation or a wireless access point. The communication device 30 may be awireless relay station, or an on-road base station device, such as aroad side unit (RSU).

Unlike the communication device 40, the communication device 30 islocated on the ground. The communication device 30 can also be called aground station device. The communication device 30 may be a deviceitself located on the ground, or may be a device mounted on an object(for example, a structure or a device) located on the ground. Forexample, the communication device 30 may be a communication devicedisposed on a structure on the ground, or may be a communication devicedisposed on a mobile object moving on the ground. More specifically, thecommunication device 30 may be an antenna installed on a structure, suchas a building, and a signal processing device connected to the antenna.The communication device 30 may naturally be a structure or a mobileobject itself. The term “on the ground” refers to “on the ground” in thebroad sense, including not only “on land” (narrow sense expression of“on the ground”), but also “underground”, “on water”, and “underwater”.

The communication device 30 need not be a fixed object. Thecommunication device 30 may be installed on a mobile object, such as anautomobile. The communication device 30 need not be present on land(narrow sense expression of “on the ground”), and may be an objectpresent in the air, such as an aerial vehicle, a drone, or a helicopter,or an object present on the sea or under the sea, such as a ship or asubmarine. In this case, the communication device 30 can perform thewireless communication with other fixedly installed communicationdevices.

The communication device 30 may be a structure itself, or may be adevice installed on the structure. The structure is, for example, abuilding, such as an office building, a house, a steel tower, a stationfacility, an airport facility, a harbor facility, or a stadium. Theconcept of the term “structure” includes not only a building, but also anon-building structure, such as a tunnel, a bridge, a dam, a fence, or asteel pole, and equipment, such as a crane, a gate, or a windmill. Theconcept of the term “structure” includes not only a structure on land(narrow sense expression of “on the ground”) or underground, but also awater-surface structure, such as a pier or a mega-float, and anunderwater structure, such as oceanographic observation equipment.

The communication device 30 may be a donor base station, or may be arelay station. When the communication device 30 is a relay station, thecommunication device 30 may be mounted on any device as long assatisfying a relaying function. For example, the communication device 30may be mounted on a terminal device such as a smartphone, may be mountedon an automobile or a rickshaw, may be mounted on a balloon, anairplane, or a drone, or may be mounted on a home electric appliancesuch as a television, a game machine, an air conditioner, arefrigerator, or a lighting device. Each of these devices itself maynaturally be regarded as the communication device 30.

The communication device 30 may be a fixed station, or may be a mobilestation. The mobile station is a wireless communication deviceconfigured to be movable. In this case, the communication device 30 maybe a device installed on a mobile object, or may be the mobile objectitself. For example, a relay station device having mobility can beregarded as the communication device 30 serving as the mobile station. Adevice, such as a vehicle, a drone, or a smartphone, that originally hasthe mobility and is provided with a communication function (at least anoptical wireless communication function) corresponds to thecommunication device 30 serving as the mobile station.

The mobile object may herein be a mobile terminal such as a smartphoneor a mobile phone. The mobile object may also be a mobile object (forexample, a vehicle, such as an automobile, a bicycle, a bus, a truck, atwo-wheeled motor vehicle, a train, or a maglev train) that moves onland (narrow sense expression of “on the ground”), or may be a mobileobject (such as an underground train) that moves underground (forexample, in the tunnel).

The mobile object may also be a mobile object (for example, a vessel,such as a passenger ship, a cargo ship, or a hovercraft) that moves onthe water surface, or may be a mobile object (for example, underwatervessel, such as a submersible, a submarine, or an unmanned underseavehicle) that moves underwater.

The mobile object may also be a mobile object (for example, an aerialvehicle, such as an airplane, an airship, or a drone) that moves in theatmosphere.

The communication device 30 may be a communication device that can floatin the air. The communication device 30 may be, for example, an aircraftstation device. The aircraft station device is a wireless communicationdevice, such as an aerial vehicle, that can float in the atmosphere(including the stratosphere). The aircraft station device may be adevice mounted on, for example, the aerial vehicle, or may be the aerialvehicle itself. The concept of the term “aerial vehicle” includes notonly a heavier-than-air aircraft, such as an airplane or a glider, butalso a lighter-than-air aircraft, such as a balloon or an airship. Theconcept of the term “aerial vehicle” includes not only theheavier-than-air aircraft and the lighter-than-air aircraft, but also arotorcraft, such as a helicopter or an autogyro. The aircraft stationdevice (or the aerial vehicle with the aircraft station device mountedthereon) may be an unmanned aerial vehicle such as a drone.

The concept of the term “unmanned aerial vehicle” also includes unmannedaircraft systems (UAS) and tethered UAS. The concept of the term“unmanned aerial vehicle” includes lighter-than-air (LTA) UAS andheavier-than-air (HTA) UAS. In addition, the concept of the term“unmanned aerial vehicle” also includes high-altitude UAS platforms(HAPs).

As described above, the communication device 30 may be a relay stationdevice (relay device). The relay station device is, for example, anaeronautical station or an earth station. The aeronautical station is aradio station installed on the ground or on a mobile object that moveson the ground, in order to communicate with the aircraft station device.The earth station is a radio station located on the Earth (including inthe air) in order to communicate with a satellite station device. Theearth station may be a large earth station, or may be a small earthstation such as a very small aperture terminal (VSAT).

The earth station may be a VSAT control earth station (also called“master station” or “hub station”), or may be a VSAT earth station (alsocalled “slave station”). The earth station may also be a radio stationinstalled on a mobile object that moves on the ground. Examples of theearth station mounted on a vessel include earth stations on boardvessels (ESV). The earth station may include an aircraft earth stationthat is installed on an aerial vehicle (including a helicopter) andcommunicates with the satellite station. The earth station may includean aeronautical earth station that is installed on a mobile object thatmoves on the ground and communicates with the aircraft earth stationthrough the satellite station. The relay station device may be aportable mobile radio station that communicates with the satellitestation and the aircraft station.

The communication device 30 can be used, operated, and/or managed byvarious entities. For example, the mobile network operators (MNOs), themobile virtual network operators (MVNOs), the mobile virtual networkenablers (MVNEs), the neutral host network (NHN) operators, theenterprises, the educational institutions (such as the schoolcorporations and the school boards of the local governments), themanagers of real estates (such as buildings and apartments), and theprivate persons can be assumed as the entities. The subjects to use,operate, and/or manage the communication device 30 are naturally notlimited to the above-listed entities.

The communication device 30 may be installed and/or operated by onebusiness operator, or may be installed and/or operated by one privateperson. The subjects to install and/or operate the communication device30 are naturally not limited to these subjects. For example, thecommunication device 30 may be installed and/or operated by a pluralityof business operators, or installed and/or operated by a plurality ofprivate persons. The communication device 30 may be common equipmentused by a plurality of business operators or a plurality of privatepersons. In this case, the equipment may be installed and/or operated bya third party different from the users.

Communication Device (Satellite Station)

The communication device 40 is an optical wireless communication devicethat uses the light to wirelessly communicate with the communicationdevice 30 or another communication device 40. The communication device40 is, for example, an optical communication modem. The communicationdevice 40 is not limited to the optical communication modem. Forexample, the communication device 40 may be a device corresponding to awireless base station or a wireless access point. The communicationdevice 40 may be a wireless relay station.

Unlike the communication device 30, the communication device 40 islocated outside the atmosphere (in the outer space). The communicationdevice 40 is, for example, a device (satellite station device) installedon a mobile object that moves outside the atmosphere (hereinafter,called “space moving object”). The communication device 40 is, forexample, a communication device mounted on an artificial satellite.Examples of the space moving object include artificial celestial bodies,such as the artificial satellite, a spacecraft, and a space station. Thecommunication device 40 may be the space moving object itself. Forexample, the communication device 40 may be the artificial satelliteitself. Also in this case, the communication device 40 can be regardedas the satellite station device.

The term “satellite station device” refers herein to a wirelesscommunication device that can float outside the atmosphere. Thesatellite station device may be a device mounted on the space movingobject such as the artificial satellite, or may be the space movingobject itself. The satellite serving as the satellite station device maybe any of a low earth orbiting (LEO) satellite, a medium earth orbiting(MEO) satellite, a geostationary earth orbiting (GEO) satellite, and ahighly elliptical orbiting (HEO) satellite. The satellite station devicemay naturally be a device mounted on the LEO satellite, the MEOsatellite, the GEO satellite, or the HEO satellite.

The communication device 40 can be used, operated, and/or managed byvarious entities, in the same way as the communication device 30. In thesame way as the communication device 30, the communication device 40 maybe installed and/or operated by one business operator, or may beinstalled and/or operated by one private person.

2-2. Specific Configuration Example of Communication System

FIG. 2 is a diagram illustrating a specific configuration example of thecommunication system 1.

The terminal device 10 is, for example, a client PC, and the serverdevice 20 is, for example, a server in an artificial satellite. Thecommunication device 30 is, for example, an optical communication modemserving as a ground station, and the communication device 40 is, forexample, an optical communication modem serving as a satellite station.The configuration illustrated in FIG. 2 is merely an example. Theconfiguration of the communication system 1 is not limited to theconfiguration illustrated in FIG. 2 .

In the example of FIG. 2 , each of the terminal device 10, the serverdevice 20, the communication device 30, and the communication device 40is configured to be connectable through a connector, such as an RJ45connector. The terminal device 10 is connectable to the communicationdevice 30 through a communication cable, such as an Ethernet cable. Inthe same way, the server device 20 is connectable to the communicationdevice 40 through a communication cable, such as a local area network(LAN) cable.

The communication device 30 and the communication device 40 includes afunctional block serving as a wired LAN physical layer and a functionalblock serving as an optical FEC physical layer, respectively. Thesefunctional blocks are connected together through a standard interface,such as a media-independent interface (MII).

The above has described the specific configuration example of thecommunication system 1. The following specifically describesconfigurations of the respective devices constituting the communicationsystem 1 according to the embodiment. The configurations of therespective devices to be described below are merely examples. Theconfigurations of the respective devices may differ from those describedbelow.

2-3. Configuration of Terminal Device

First, a configuration of the terminal device 10 will be described. Theterminal device 10 is the information processing device capable ofexchanging data with a communication device, such as the communicationdevice 30, having the free-space optical communication function. Theterminal device 10 may have the free-space optical communicationfunction in the same way as the communication device 30 and thecommunication device 40. In this case, the terminal device 10 can beregarded as a communication device.

FIG. 3 is a diagram illustrating a configuration example of the terminaldevice 10 according to the embodiment of the present disclosure. Theterminal device 10 includes a communication interface 11, a storage unit12, and a controller 13. The configuration illustrated in FIG. 3 is afunctional configuration, and hardware may have a differentconfiguration therefrom. Functions of the terminal device 10 may beimplemented so as to be distributed over a plurality of physicallyseparated components.

The communication interface 11 is an interface for communicating withanother device (for example, the communication device 30). Thecommunication interface 11 may be a network interface, or may be adevice connection interface. For example, the communication interface 11may be a local area network (LAN) interface, such as a network interfacecard (NIC), or may be a Universal Serial Bus (USB) interface constitutedby, for example, a USB interface host controller and a USB port. Thecommunication interface 11 may be a wired interface, or may be awireless interface. The communication interface 11 serves as acommunication unit for the terminal device 10.

The storage unit 12 is a data readable/writable storage device, such asa dynamic random access memory (DRAM), a static random access memory(SRAM), a flash memory, or a hard disk. The storage unit 12 serves as astorage unit for the terminal device 10.

The controller 13 is a controller that controls components of theterminal device 10. The controller 13 is implemented by a processor,such as a central processing unit (CPU) or a microprocessing unit (MPU).For example, the processor uses, for example, a random-access memory(RAM) as a work area to execute various computer programs stored in thestorage device in the terminal device 10 to implement the controller 13.The controller 13 may be implemented by an integrated circuit, such asan application specific integrated circuit (ASIC) or afield-programmable gate array (FPGA). Any of the CPU, the MPU, the ASIC,and the FPGA can be regarded as the controller.

The controller 13 may have the same functions as those of either one orboth of the controllers of the communication device 30 and thecommunication device 40 to be described later. For example, thecontroller 13 may include functional blocks that perform the sameoperations as those of functional blocks (blocks raging from anacquisition unit to a reception controller) constituting the controllerof the communication device 30.

2-4. Configuration of Server Device

The following describes a configuration of the server device 20. Theserver device 20 is the information processing device capable ofexchanging data with a communication device, such as the communicationdevice 40, having the free-space optical communication function. Theserver device 20 may have the free-space optical communication functionin the same way as the communication device 30 and the communicationdevice 40. In this case, the server device 20 can be regarded as acommunication device.

FIG. 4 is a diagram illustrating a configuration example of the serverdevice 20 according to the embodiment of the present disclosure. Theserver device 20 includes a communication interface 21, a storage unit22, and a controller 23. The configuration illustrated in FIG. 4 is afunctional configuration, and hardware may have a differentconfiguration therefrom. Functions of the server device 20 may beimplemented so as to be distributed over a plurality of physicallyseparated components.

The communication interface 21 is an interface for communicating withanother device (for example, the communication device 40). Thecommunication interface 21 may be a network interface, or may be adevice connection interface. For example, the communication interface 21may be a LAN interface, such as a NIC, or may be a USB interfaceconstituted by, for example, a USB host controller and a USB port. Thecommunication interface 21 may be a wired interface, or may be awireless interface. The communication interface 21 serves as acommunication unit for the server device 20.

The storage unit 22 is a data readable/writable storage device, such asa DRAM, an SRAM, a flash memory, or a hard disk. The storage unit 22serves as a storage unit for the server device 20.

The controller 23 is a controller that controls components of the serverdevice 20. The controller 23 is implemented by a processor, such as aCPU or an MPU. For example, the processor uses, for example, a RAM as awork area to execute various computer programs stored in the storagedevice in the server device 20 to implement the controller 23. Thecontroller 23 may be implemented by an integrated circuit, such as anASIC or an FPGA. Any of the CPU, the MPU, the ASIC, and the FPGA can beregarded as the controller.

The controller 23 may have the same functions as those of either one orboth of the controllers of the communication device 30 and thecommunication device 40 to be described later. For example, thecontroller 23 may include functional blocks that perform the sameoperations as those of functional blocks (blocks raging from anacquisition unit to a reception controller) constituting the controllerof the communication device 40.

2-5. Configuration of Communication Device (Ground Station)

The following describes a configuration of the communication device 30.The communication device 30 is the optical wireless communication devicethat uses the light to wirelessly communicate with the communicationdevice 40 or another communication device 30.

FIG. 5 is a diagram illustrating a configuration example of thecommunication device 30 according to the embodiment of the presentdisclosure. The communication device 30 includes an optical spacecommunication unit 31, a communication interface 32, a storage unit 33,and a controller 34. The configuration illustrated in FIG. 5 is afunctional configuration, and hardware may have a differentconfiguration therefrom. Functions of the communication device 30 may beimplemented so as to be distributed over a plurality of physicallyseparated devices.

The optical space communication unit 31 is a communication interface forperforming the free-space optical communication with anothercommunication device (for example, the communication device 40 oranother communication device 30) having the free-space opticalcommunication function. A transmission medium used for the free-spaceoptical communication by the optical space communication unit 31 is notlimited to the visible light, and may be the infrared light. The lightused as the transmission medium by the optical space communication unit31 may be high-directivity light, such as the laser light. The lightused as the transmission medium by the optical space communication unit31 may naturally be low-directivity light, such as the Radial light.

The optical space communication unit 31 includes a reception processor311 and a transmission processor 312. The reception processor 311performs, for example, signal processing of an optical signal receivedvia, for example, a photosensor. The transmission processor 312performs, for example, signal processing to convert the transmit datainto an optical signal. The optical space communication unit 31 mayinclude a plurality of the reception processors 311 and a plurality ofthe transmission processors 312. When the optical space communicationunit 31 supports a plurality of wireless access schemes, the processorsof the optical space communication unit 31 can be individuallyconfigured for each of the wireless access schemes. For example, whenthe communication device 30 supports a plurality of wireless accessschemes having different error correction methods, the receptionprocessor 311 and the transmission processor 312 may be individuallyconfigured for each of the wireless access schemes.

The communication interface 32 is an interface for communicating withanother device (for example, the terminal device 10). The communicationinterface 32 may be a network interface, or may be a device connectioninterface. For example, the communication interface 32 may be a LANinterface, such as a NIC, or may be a USB interface constituted by, forexample, a USB host controller and a USB port. The communicationinterface 32 may be a wired interface, or may be a wireless interface.The communication interface 32 serves as a communication unit for thecommunication device 30.

The storage unit 33 is a data readable/writable storage device, such asa DRAM, an SRAM, a flash memory, or a hard disk. The storage unit 33serves as a storage unit for the communication device 30. The storageunit 33 stores, for example, an error correction block and receiveddata.

The controller 34 is a controller that controls components of thecommunication device 30. The controller 34 is implemented by aprocessor, such as a CPU or an MPU. For example, the processor uses, forexample, a RAM as a work area to execute various computer programsstored in the storage device in the communication device 30 to implementthe controller 34. The controller 34 may be implemented by an integratedcircuit, such as an ASIC or an FPGA. Any one of the CPU, the MPU, theASIC, and the FPGA can be regarded as the controller.

As illustrated in FIG. 5 , the controller 34 includes an acquisitionunit 341, a shaping unit 342, an error correction unit 343, aninterleaving unit 344, a transmission controller 345, a detection unit346, a deinterleaving unit 347, and a reception controller 348. Each ofthe blocks (acquisition unit 341 to reception controller 348)constituting the controller 34 is a functional block representing afunction of the controller 34. These functional blocks may be softwareblocks, or may be hardware blocks. For example, each of theabove-described functional blocks may be one software module implementedby software (including a microprogram), or may be one circuit block on asemiconductor chip (die). Each of the functional blocks may naturally beone processor or one integrated circuit. Any method may be used toconfigure the functional blocks. The controller 34 may be configured infunctional units different from the above-described functional blocks.

As described above, the controller 13 of the terminal device 10 and thecontroller 23 of the server device 20 may each include the functionalblocks included in the controller 34 of the communication device 30. Inthis case, the term “communication device 30” to be mentioned in thefollowing description can be replaced with “terminal device 10” or“server device 20” as appropriate. Each of the terms “controller 34” and“acquisition unit 341” to “reception controller 348” to be mentioned inthe following description can also be replaced with “controller 13” or“controller 23” as appropriate.

The controller 34 may have the same functions as those of a controllerof the communication device 40 to be described later. For example, theoperations of the functional blocks (acquisition unit 341 to receptioncontroller 348) of the controller 34 may be the same as those offunctional blocks (acquisition unit to reception controller)constituting the controller of the communication device 40. In thiscase, the terms representing the functional blocks of the controller 34(“acquisition unit 341” to “reception controller 348”) to be mentionedin the following description can be replaced with terms representing thefunctional blocks of the controller of the communication device 40 asappropriate.

2-6. Configuration of Communication Device (Satellite Station)

The following describes a configuration of the communication device 40.The communication device 40 is the optical wireless communication devicethat uses the light to wirelessly communicate with the communicationdevice 30 or another communication device 40.

FIG. 6 is a diagram illustrating a configuration example of thecommunication device 40 according to the embodiment of the presentdisclosure. The communication device 40 includes an optical spacecommunication unit 41, a communication interface 42, a storage unit 43,and a controller 44. The configuration illustrated in FIG. 6 is afunctional configuration, and hardware may have a differentconfiguration therefrom. Functions of the communication device 40 may beimplemented so as to be distributed over a plurality of physicallyseparated devices.

The optical space communication unit 41 is a communication interface forperforming the free-space optical communication with anothercommunication device (for example, the communication device 30 oranother communication device 40) having the free-space opticalcommunication function. The transmission medium used for the free-spaceoptical communication by the optical space communication unit 41 is notlimited to the visible light, and may be the infrared light. The lightused as the transmission medium by the optical space communication unit41 may be high-directivity light, such as the laser light. The lightused as the transmission medium by the optical space communication unit41 may naturally be high-directivity light, such as the synchrotronradiation.

The optical space communication unit 41 includes a reception processor411 and a transmission processor 412. The reception processor 411performs, for example, the signal processing of an optical signalreceived via, for example, a photosensor. The transmission processor 412performs, for example, the signal processing to convert the transmitdata into an optical signal. The optical space communication unit 41 mayinclude a plurality of the reception processors 411 and a plurality ofthe transmission processors 412. When the optical space communicationunit 41 supports a plurality of wireless access schemes, the processorsof the optical space communication unit 41 can be individuallyconfigured for each of the wireless access schemes. For example, whenthe communication device 40 supports a plurality of wireless accessschemes having different error correction methods, the receptionprocessor 411 and the transmission processor 412 may be individuallyconfigured for each of the wireless access schemes.

The communication interface 42 is an interface for communicating withanother device (for example, the server device 20). The communicationinterface 42 may be a network interface, or may be a device connectioninterface. For example, the communication interface 42 may be a LANinterface, such as a NIC, or may be a USB interface constituted by, forexample, a USB host controller and a USB port. The communicationinterface 42 may be a wired interface, or may be a wireless interface.The communication interface 42 serves as a communication unit for thecommunication device 40.

The storage unit 43 is a data readable/writable storage device, such asa DRAM, an SRAM, a flash memory, or a hard disk. The storage unit 43serves as a storage unit for the communication device 40. The storageunit 43 stores, for example, an error correction block and receiveddata.

The controller 44 is a controller that controls components of thecommunication device 40. The controller 44 is implemented by aprocessor, such as a CPU or an MPU. For example, the processor uses, forexample, a RAM as a work area to execute various computer programsstored in a storage device in the communication device 40 to implementthe controller 44. The controller 44 may be implemented by an integratedcircuit, such as an ASIC or an FPGA. Any of the CPU, the MPU, the ASIC,and the FPGA can be regarded as the controller.

As illustrated in FIG. 6 , the controller 44 includes an acquisitionunit 441, a shaping unit 442, an error correction unit 443, aninterleaving unit 444, a transmission controller 445, a detection unit446, a deinterleaving unit 447, and a reception controller 448. Each ofthe blocks (acquisition unit 441 to reception controller 448)constituting the controller 44 is a functional block representing afunction of the controller 44. These functional blocks may be softwareblocks, or may be hardware blocks. For example, each of theabove-described functional blocks may be one software module implementedby software (including a microprogram), or may be one circuit block on asemiconductor chip (die). Each of the functional blocks may naturally beone processor or one integrated circuit. Any method may be used toconfigure the functional blocks. The controller 44 may be configured infunctional units different from the above-described functional blocks.

As described above, the controller 13 of the terminal device 10 andcontroller 23 of the server device 20 may each include the functionalblocks included in the controller 44 of the communication device 40. Inthis case, the term “communication device 40” to be mentioned in thefollowing description can be replaced with “terminal device 10” or“server device 20” as appropriate. Each of the terms “controller 44” and“acquisition unit 441” to “reception controller 448” to be mentioned inthe following description can also be replaced with “controller 13” or“controller 23” as appropriate.

The controller 44 may have the same functions as those of the controller34 of the communication device 30 described above. For example, theoperations of the functional blocks (acquisition unit 441 to receptioncontroller 448) of the controller 44 may be the same as those of thefunctional blocks (acquisition unit 341 to reception controller 348)constituting the controller 34 of the communication device 30. In thiscase, the terms representing the functional blocks of the controller 44(“acquisition unit 441” to “reception controller 448”) to be mentionedin the following description can be replaced with the terms representingthe functional blocks of the controller 34 of the communication device30 (“acquisition unit 341” to “reception controller 348”) asappropriate. The terms representing the functional blocks of thecontroller 34 (“acquisition unit 341” to “reception controller 348”) tobe mentioned in the following description may naturally be replaced withthe terms representing the functional blocks of the controller 44 of thecommunication device 40 (“acquisition unit 441” to “reception controller448”).

2-7. Relation in Physical Layer Configuration Between OSI ReferenceModel and Communication Device

In the example of FIG. 2 , each of the communication device 30 and thecommunication device 40 is, for example, a free-space optical modem, andhas the wired LAN physical layer and the optical FEC layer. Thefollowing describes a relation in physical layer configuration betweenthe OSI reference model and the communication device (either one or bothof the communication device 30 and the communication device 40). In thissection (2-7), each of the communication device 30 and the communicationdevice 40 is the free-space optical modem, but is naturally not limitedto the free-space optical modem.

In the following description, the term “Ethernet” refers not only toEthernet (registered trademark) as a registered trademark, but also toEthernet in the broad sense as a standard, such as IEEE 802.3. The term“Ethernet” to be mentioned in the following description can naturally bereplaced with, for example, “IEEE 802.3”, “IEEE 802.3 Ethernet”,“standard network interface”, “wired LAN”, or “Ethernet as a standard”.

FIG. 7 is a diagram illustrating the relation in physical layerconfiguration between the OSI reference model and the free-space opticalmodem. The free-space optical modem (each of the communication device 30and the communication device 40) of the present embodiment correspondsto the physical layer of the OSI reference model. The IEEE 802.3standard as an Ethernet standard defines, for example, themedia-independent interface (MII), the gigabit media-independentinterface (GMII), the 10-gigabit media-independent interface (XGMII),and the 10-gigabit attachment unit interface (XAUI) as themedia-independent interface standard for connecting a media accesscontrol (MAC) layer to the physical layer. The free-space optical modemcan serve as an Ethernet media converter by connecting the optical FECphysical layer through this interface, and using existing higher-levelEthernet layers as they are.

The optical FEC physical layer included in the free-space optical modemof the present embodiment corresponds to, for example, the PhysicalCoding Sublayer (PCS) and the Physical Medium Attachment (PMA). ThePhysical Medium Dependent (PMD) illustrated in FIG. 7 corresponds to,for example, a laser or light modulator, a transmission/receptionoptical system, or a photodetector.

The embodiment to be described below is targeted at a 100 Mbps network(for example, 100BASE-TX) as an example. However, the target ofapplication of the present embodiment is not limited to the 100 Mbpsnetwork, and may be, for example, a 1 Gbps network (for example,1000BASE-T). When the free-space optical modem of the present embodimentsupports the 1 Gbps network (for example, 1000BASE-T), the optical FECphysical layer may be connected to the higher-level layers through theGMII, as illustrated on the right side in FIG. 7 . In order to improveerror correction capability and redundancy, the parity length or thesymbol length of a Reed-Solomon code may be increased, or the datamodulation method may be replaced with, for example, 64b/66b encoding.Even in a case where the free-space optical modem of the presentembodiment is required to support a higher-speed network such as a 10Gbps network, as long as the media-independent interface conforms to theEthernet standard, the free-space optical modem of the presentembodiment can serve as the Ethernet media converter even if the errorcorrection method or the block structure is greatly changed.

3. ERROR CORRECTION BLOCK FORMAT

The communication devices 30 and 40 of the present embodiment areconfigured to be capable of changing the interleaving length. Thecommunication device 30 or the communication device 40 changes theinterleaving length according to the characteristics of the transmissionspace so as to achieve the stable free-space optical communication. Thepresent embodiment employs a data format making the interleaving lengthdetectable so as to allow the receiving side of the free-space opticalcommunication (for example, the communication device 40) to detect theinterleaving length used by the transmission side of the free-spaceoptical communication (for example, the communication device 30).

In the present embodiment, as an example, the data format is an errorcorrection block format. The following describes the error correctionblock format of the present embodiment. A basic error correction blockformat will first be described. An extended format in the presentembodiment will then be described.

3-1. Basic Configuration

First, the basic error correction block format will be described. In thefollowing description, the basic error correction block may be called abasic block. The block format of the basic block is constituted by theerror correction block encoded using a product code. FIGS. 8 and 9 arediagrams illustrating examples of the block format of the basic block.The block format illustrated in FIGS. 8 and 9 is a forward errorcorrection format for the free-space optical communication using aReed-Solomon product code. In the following description, thelongitudinal direction of the figure is called the vertical direction,and the lateral direction of the figure is called the horizontaldirection.

As illustrated in FIGS. 8 and 9 , the basic block has a size of 240bytes ((2+230+8) bytes) in the horizontal direction and 253 bytes in thevertical direction. A head in the horizon direction (that is, a blockboundary) of the basic block is provided with a synchronization code(hereinafter, called a SYNC code). In the example of FIG. 8 , two bytesin the horizontal direction serve as the SYNC code. The basic blockincludes a parity inner (PI) code of eight bytes in the horizontaldirection and a parity outer (PO) code of 32 bytes in the verticaldirection. An area of 230 bytes interposed between the SYNC code and thePO parity serves as an interleaved block. The interleaved block includessub-data of one byte in the vertical direction. An area of 50.6kilobytes obtained by removing the sub-data and the PO from theinterleaved block serves as user data (payload data).

The sub-data is data used for exchanging control information between thecommunication devices, such as the free-space optical modems. Thecommunication system of the present embodiment uses the sub-datasuperimposed in addition to the payload data so as to be capable ofcommunicating various control signals between the optical modems. Forexample, a receiver can notify a transmitter of a reception state of,for example, an error rate, so that the transmitter can change theinterleaving length for transmission to select an interleaving lengthoptimal for the transmission space. In addition, the transmission sidecan monitor a received light intensity to perform control toautomatically adjust the transmitted light output or correct thetransmitted light axis. In this way, using the sub-data transmissionenables negotiation between devices, optimization of control, and systemoperation of, for example, various monitors.

During data transmission, as illustrated in FIG. 8 , the errorcorrection block of the present embodiment receives data in the verticaldirection, and outputs the transmit data in the horizontal direction.During data reception, as illustrated in FIG. 9 , the received data isreceived in the horizontal direction, and PO decoding is performed inthe vertical direction. Therefore, the interleaving length (230 bytes inthe examples of FIGS. 8 and 9 ) increases with the increase in thelength in the horizontal direction of the interleaved block.

The Reed-Solomon product code having the PO parity and the PI parityexcels in burst error correction capacity, and can further be improvedin the error correction capacity by performing erasure correction andrepetitive correction, thus being also effective for correcting theburst errors caused by the atmospheric disturbances likely to occurduring the free-space optical communication.

The block format illustrated in FIGS. 8 and 9 is merely an example. Theerror correction block format is not limited to the format illustratedin FIGS. 8 and 9 . The specific values illustrated in FIGS. 8 and 9 canbe changed as appropriate.

3-2. Extended Configuration

The extended format in the present embodiment will be described. Theextended format described below is an error correction block format inwhich the interleaving length is extended. FIGS. 10 and 11 are diagramsillustrating the error correction block format when the interleavinglength has been extended.

The extended format is a format of a structure obtained by arranging thebasic blocks illustrated in FIGS. 8 and 9 in the horizontal direction.In the examples of FIGS. 10 and 11 , the basic block is extended to fourblocks. In the examples of FIGS. 10 and 11 , the interleaving length isfour times that of the basic block. As a result, as illustrated in FIG.11 , the deinterleaving disperses the burst errors (each marked by X)across four blocks. This dispersion can also expand the length of bursterrors correctable by the PO parity by four times. Increasing the numberof the connected blocks can further increase the interleaving length.

While increasing the interleaving length improves resistance to theburst errors, a memory capacity required for the interleaving increases,and a waiting time (latency) of the transmission and reception increasesin proportion thereto. Therefore, a mechanism is required to select theoptimal interleaving length according to the characteristics of thespace (transmission path) through which the actual communication isperformed.

The transmission-side communication device can easily change theinterleaving length using a mechanism, such as the structure illustratedin FIGS. 10 and 11 , that laterally extends the blocks by arranging anynumber of the basic blocks.

Regarding the SYNC code, to allow the receiving side to detect theinterleaving length, the transmission-side communication device assignsa special SYNC code (first synchronization code) to the SYNC code in thehead block, and assigns a SYNC code (second synchronization code)different from that in the head block to each of the other blocks. Thefirst synchronization code is a code representing a start point or anend point of the interleaving, and the second synchronization code is acode representing a continuation point of the interleaving.

As a result, the receiving-side communication device can detect theinterleaving length used by the transmission side based on a receivinginterval of the head SYNC (second synchronization code). This detectionallows the receiving-side communication device to automatically matchthe interleaving length thereof with the interleaving length of thetransmission side. As another implementation example, a block number canbe embedded in the SYNC code to allow the interleaving length to bedetected. Also in this case, the SYNC code with the number representingthe head SYNC embedded therein may be regarded as the first code, and aSYNC code with another number embedded therein may be regarded as thesecond code.

Even when the code of the error correction block format is aconvolutional code different from the Reed-Solomon product code of theblock format, the interleaving length can be detected from the receivedSYNC codes, and the interleaving length can be automatically adjusted.

The format illustrated in FIGS. 10 and 11 is merely an example. Theerror correction block format is not limited to the format illustratedin FIGS. 10 and 11 . The specific values illustrated in FIGS. 10 and 11can be changed as appropriate. The number of the arranged basic blocksis naturally not limited to four, and can be changed as appropriate.

4. OPERATIONS OF COMMUNICATION SYSTEM

The following describes operations of the communication system 1.

4-1. Basic Operation

A basic operation of the communication system 1 will first be described.FIG. 12 is a diagram for explaining the basic operation of thecommunication system 1.

In the following description, the terminal device 10 is located on theground, and the server device 20 is located in the artificial satellite.The terminal device 10 is connected to the communication device 30through the communication cable, and the server device 20 is connectedto the communication device 40 through the communication cable. Thecommunication device 30 and the communication device 40 are, forexample, the free-space optical communication modems. The followingdescribes a case where the terminal device 10 requests data from theserver device 20 using the Transmission Control Protocol/InternetProtocol (TCP/IP) suite.

After the terminal device 10 transmits a TCP packet (GET request) to thecommunication cable such as the Ethernet cable, the wired LAN physicallayer in the communication device 30 receives the packet (Step S11). Thewired LAN physical layer transmits the packet through themedia-independent interface (MII, GMII, XGMII, or XAUI) to the opticalFEC physical layer in the communication device 30 (Step S12). Theoptical FEC physical layer appends an error correction parity to thereceived data, and modulates a serial signal to be output after beingsubjected to, for example, the interleaving processing (Step S13). Theoptical FEC physical layer transmits output light generated through themodulation toward the distant satellite station (Step S14).

The communication device 40 in the satellite uses a reception detectorin the optical FEC physical layer to detect the modulated light outputfrom the communication device 30 on the ground. The optical FEC physicallayer in the communication device 40 converts the detected data intoparallel data, and then performs the deinterleaving and the errorcorrection processing (Step S15). The optical FEC physical layertransmits the error-corrected data (TCP packet) through themedia-independent interface to the wired LAN physical layer in thecommunication device 40 (Step S16). The wired LAN physical layerdelivers the TCP packet to the server device 20 connected to the wiredLAN physical layer through the communication cable (Step S17).

The server that has received the TCP packet (GET request) transmits thedata requested by the terminal device 10 to the communication device 40.Then, the wired LAN physical layer in the communication device 40receives the data (Step S21). The wired LAN physical layer transmits thedata through the media-independent interface to the optical FEC physicallayer in the communication device 40 (Step S22). The optical FECphysical layer appends the error correction parity to the received data,and modulates the serial signal to be output after being subjected to,for example, the interleaving processing (Step S23). The optical FECphysical layer transmits the output light generated through themodulation toward the distant ground station (Step S24).

The communication device 30 located on the ground uses the receptiondetector in the optical FEC physical layer to detect the modulated lightoutput from the communication device 40 in the satellite. The opticalFEC physical layer in the communication device 30 converts the detecteddata into the parallel data, and then performs the deinterleaving andthe error correction processing (Step S25). The optical FEC physicallayer transmits the error-corrected data through the media-independentinterface to the wired LAN physical layer in the communication device 30(Step S26). The wired LAN physical layer delivers the data from theserver device 20 to the terminal device 10 connected to the wired LANphysical layer through the communication cable (Step S27). Thus, theterminal device 10 can receive the desired data.

This connection configuration through the free-space opticalcommunication causes the communication device 30 and the communicationdevice 40 to serve as the Ethernet media converters, and when viewedfrom the terminal device 10 and the server device 20, the ground looksas if being connected to the satellite through a wired Ethernet link.This configuration allows the terminal device 10 and the server device20 to perform the bidirectional data communication.

4-2. Transmit Data Processing

The following describes transmit data processing performed by theoptical FEC physical layer. FIG. 13 is a diagram for explaining thetransmit data processing of the present embodiment. The transmit dataprocessing to be described below is, for example, the processing at StepS13 or S23 illustrated in FIG. 12 .

When the communication device 30 serves as the transmission-sidecommunication device, the transmit data processing is performed by, forexample, the acquisition unit 341, the shaping unit 342, the errorcorrection unit 343, the interleaving unit 344, and the transmissioncontroller 345. When the communication device 40 serves as thetransmission-side communication device, the transmit data processing isperformed by, for example, the acquisition unit 441, the shaping unit442, the error correction unit 443, the interleaving unit 444, and thetransmission controller 445.

In the following description, although the communication device 30serves as the transmission-side communication device, thetransmission-side communication device may be the communication device40. In that case, the terms, for example, “acquisition unit 341”,“shaping unit 342”, “error correction unit 343”, “interleaving unit344”, and “transmission controller 345” to be mentioned in the followingdescription can be replaced with, for example, “acquisition unit 441”,“shaping unit 442”, “error correction unit 443”, “interleaving unit444”, and “transmission controller 445”.

The following describes the transmit data processing with reference toFIG. 13 .

The acquisition unit 341 acquires the data output from the wired LANphysical layer through the media-independent interface, and supplies thedata to a first-in-first-out unit (FIFO) (Step S101). The shaping unit342 embeds an inter-frame gap (IFG) signal in each frame boundary, andthen appends a predetermined number of bytes (for example, one byte) ofthe sub-data to be used for control between the communication devices(Step S102). The error correction unit 343 appends a predeterminednumber of bytes (for example, 32 bytes) of the PO parity serving as theReed-Solomon code (Step S103).

Then, the interleaving unit 344 determines the interleaving length ofthe transmit data to be transmitted through the free-space opticalcommunication. The interleaving unit 344 determines the interleavinglength based on information on the transmission path of the free-spaceoptical communication to be used for transmitting the transmit data. Theinformation on the transmission path is, for example, informationindicating whether the free-space optical communication is communicationperformed between the ground station and the satellite station orcommunication performed between the satellite stations. The informationon the transmission path may include information indicating whether thefree-space optical communication is communication performed between theground stations.

In the case where the interleaving length is determined based on theinformation on the transmission path, for example, when the transmissionpath is between the satellite stations, the interleaving unit 344determines the interleaving length to be a first interleaving length(for example, an interleaving length for one block). When thetransmission path is between the ground station and the satellitestation, the burst errors are considered to be likely to occur.Therefore, the interleaving unit 344 determines the interleaving lengthto be a second interleaving length (for example, an interleaving lengthfor four blocks) longer than the first interleaving length. When thetransmission path is between the ground stations, the interleaving unit344 determines the interleaving length to be a third interleaving length(for example, an interleaving length for two blocks) longer than thefirst interleaving length and shorter than the second interleavinglength. The interleaving length is not limited to these examples, andmay have various values. The transmission path is also not limited tobetween the satellite stations, between the ground station and thesatellite station, or between the ground stations.

The information used by the interleaving unit 344 to determine theinterleaving length is not limited to the information on thetransmission path. The interleaving unit 344 may determine theinterleaving length based on, for example, information on the error rateduring the communication with the receiving side of the free-spaceoptical communication. For example, if the error rate is lower than apredetermined threshold, the interleaving unit 344 determines theinterleaving length to be a fourth interleaving length (for example, aninterleaving length for one block). If, in contrast, the error rate ishigher than the predetermined threshold, the optical communication isconsidered to be that performed in a transmission space in which anerror is unlikely to occur. Therefore, the interleaving unit 344determines the interleaving length to be a fifth interleaving length(for example, an interleaving length for four blocks) longer than thefourth interleaving length. The interleaving length is not limited tothese examples, and may have various values. A plurality of thepredetermined thresholds may be provided, and a plurality of theinterleaving lengths may be prepared in accordance with the error rates.

The information used for determining the interleaving length, such asthe information on the transmission path or the information on the errorrate, may be acquired from the sub-data included in the data receivedfrom the other communication device. For example, the acquisition unit341 may acquire the sub-data included in the error correction block, andthe interleaving unit 344 may determine the interleaving length based onthe information (for example, the error rate) included in the sub-dataacquired by the acquisition unit 341.

The interleaving unit 344 subsequently interleaves the transmit databased on the determined interleaving length (Step S104).

Then, the shaping unit 342 shapes the interleaved transmit data so as tomake the interleaving length detectable on the receiving side of thefree-space optical communication. For example, the shaping unit 342appends the PI parity and the SYNC data to the interleaved transmit data(Step S105, S106). At this time, as described in the above section “3-2.Extended Configuration”, the shaping unit 342 disposes the firstsynchronization code at a place representing the start point (or the endpoint) of the interleaving and the second synchronization code at aplace representing the continuation point of the interleaving.

Then, the transmission controller 445 performs 8b/10b modulation on thetransmit data (Step S107). In addition, the transmission controller 445performs parallel-to-serial conversion on the data subjected to the8b/10b modulation (Step S108). As a result, a modulation signalincluding a smaller amount of direct-current (DC) component is suppliedto an optical modulation unit, and the transmission controller 445 canoutput modulated transmission light. An encoding scheme other than the8b/10b encoding may be used, such as a 64b/66b encoding scheme.Otherwise, for example, a randomizer (scrambler) may be used.

4-3. Received Data Processing

The following describes received data processing performed by theoptical FEC physical layer. FIG. 14 is a diagram for explaining thereceived data processing of the present embodiment. The received dataprocessing to be described below is, for example, the processing at StepS15 or S25 illustrated in FIG. 12 .

When the communication device 40 serves as the receiving-sidecommunication device, the received data processing is performed by, forexample, the detection unit 446, the deinterleaving unit 447, and thereception controller 448. When the communication device 30 serves as thereceiving-side communication device, the received data processing isperformed by, for example, the detection unit 346, the deinterleavingunit 347, and the reception controller 348.

In the following description, although the communication device 40serves as the receiving-side communication device, the receiving-sidecommunication device may be the communication device 30. In that case,the terms, for example, “detection unit 446”, “deinterleaving unit 447”,and “reception controller 448” to be mentioned in the followingdescription can be replaced with, for example, “detection unit 346”,“deinterleaving unit 347”, and “reception controller 348”.

The following describes the received data processing with reference toFIG. 14 .

The reception controller 448 uses a phase-locked loop (PLL) circuit toextract a synchronizing clock from a serial data string received by thephotodetector (Step S201), and converts the serial data into paralleldata (Step S202). The reception controller 448 then performs 8b/10bdemodulation (Step S203).

The received data has been shaped such that the interleaving length isdetectable therefrom. Specifically, the first synchronization code isdisposed at the place representing the start point (or the end point) ofthe interleaving, and the second synchronization code is disposed at theplace representing the continuation point of the interleaving. Thedetection unit 346 detects the SYNC codes to identify the interleavinglength (Step S204).

The deinterleaving unit 447 uses a PI correction circuit to perform theerror correction on the demodulated data (Step S205), and performs thedeinterleaving processing based on the interleaving length detected bythe detection unit 346 (Step S206). Then, the reception controller 448performs PO correction (Step S207). The deinterleaving disperses theburst errors, and thereby can increase the capacity of the burst errorcorrection using the PO parity.

The reception controller 448 then extracts the sub-data to detect theframe boundary (IFG) (Step S208). The reception controller 448 uses theFIFO to remove a clock frequency error (Step S209). The receptioncontroller 448 transmits the received data to the wired LAN physicallayer.

5. MODIFICATIONS

The above-described embodiment merely illustrates an example, andvarious modifications and applications thereof can be made.

5-1. Modifications in Terms of Processing

For example, in the above-described embodiment, the free-space opticalcommunication performed by the communication devices is thecommunication performed between the ground station and the satellitestation (for example, the communication performed between thecommunication device 30 and the communication device 40). However, thefree-space optical communication may be the communication performedbetween the satellite stations (for example, communication performedbetween the communication devices 40). The free-space opticalcommunication may be the communication performed between the groundstations (for example, communication performed between the communicationdevices 30).

In the above-described embodiment, the data format configured to makethe interleaving length detectable is the error correction block format,but is not limited to the error correction block format, and may beanother data format constituted by one or a plurality of data blocks. Inthat case, SYNC codes, such as the first synchronization code and thesecond synchronization code, may be appended to the data block orblocks.

In the above-described embodiment, the error correction block format isthat of the error correction block encoded using the Reed-Solomonproduct code. However, the encoding scheme is not limited to that usingthe Reed-Solomon product code. The encoding scheme may use anotherproduct code. The code used in the encoding scheme is not limited to theproduct code, and may be another block code.

In the above-described embodiment, an Ethernet frame directly serves asthe payload data. However, the Ethernet frame may be replaced with aGeneric Framing Procedure (GFP) frame (ITU-T RecommendationG.7041/Y.1303). As a result, since the GFP frame can accommodate variousclient signals including the Ethernet signals, versatility of thefree-space optical communication network can be increased.

5-2. Modifications in Terms of Device Configuration

In the above-described embodiment, the terminal device 10 and the serverdevice 20 exchange information through the free-space opticalcommunication. However, the devices that exchange information throughthe free-space optical communication are not limited to the terminaldevice 10 and the server device 20. Information processing devices otherthan the terminal device 10 and the server device 20 may exchangeinformation. For example, a sensor device and another sensor device mayexchange information through the free-space optical communication.

The terminal device 10 and another terminal device 10 may exchangeinformation through the free-space optical communication. The serverdevice 20 and another server device 20 may exchange information throughthe free-space optical communication.

In the above-described embodiment, the terminal device 10 is located onthe ground, and the server device 20 is located in the outer space.However, the server device 20 may be located on the ground, and theterminal device 10 may be located in the outer space. For example, theserver device 20 may be located in a base station on the ground, and theterminal device 10 may be located in a space station.

5-3. Other Modifications

A control device for controlling each of the terminal device 10, theserver device 20, the communication device 30, and the communicationdevice 40 may be implemented by a dedicated computer system or ageneral-purpose computer system.

For example, a communication program for executing the above-describedoperations (for example, the transmission/reception processing) isdelivered by being stored in a computer-readable recording medium, suchas an optical disc, a semiconductor memory, a magnetic tape, or aflexible disk. The communication program is installed, for example, on acomputer, and executes the above-described processing to configure thecontrol device. In that case, the control device may be a device (forexample, a personal computer) outside the terminal device 10, the serverdevice 20, the communication device 30, or the communication device 40.Alternatively, the control device may be a device (for example, thecontroller 13, 23, 34, or 44) inside the terminal device 10, the serverdevice 20, the communication device 30, or the communication device 40.

The above-described communication program may be stored on a disk deviceprovided on a server device on a network such as the Internet so as tobe, for example, downloadable to the computer. The above-describedfunctions may be provided by cooperation between an operating system(OS) and application software. In that case, parts other than the OS maybe delivered by being stored in a medium, or the parts other than the OSmay be stored in the server device so as to be, for example,downloadable to the computer.

All or some of the processes described in the above embodiment that havebeen described as being automatically executed can be manually executed,or all or some of the processes described as being manually executed canbe automatically executed using known methods. In addition, theinformation including procedures, specific names, various types of data,and parameters illustrated in the document and the drawings can befreely changed unless otherwise specified. For example, the various typeof information illustrated in the drawings are not limited to thoseillustrated in the drawings.

The illustrated components of the devices are merely functionallyconceptual, and need not be physically configured as illustrated in thedrawings. In other words, the specific mode of distribution andintegration of the devices is not limited to those illustrated in thedrawings, and all or some of the devices can be configured in afunctionally or physically distributed or integrated manner in any unitsaccording to various types of loads or use conditions.

The above-described embodiment can be combined as appropriate within arange not causing contradiction in processing details. The order of thesteps illustrated in each of the flowcharts and the sequence diagrams inthe above-described embodiment can be changed as appropriate.

For example, the present embodiment can also be implemented as any ofall components constituting the device or the system, including, forexample, a processor as, for example, a system large-scale integrated(LSI) circuit, a module using a plurality of the processors, a unitusing a plurality of the modules, or a set obtained by further addingother functions to the unit (that is, some components of the device).

In the present embodiment, the term “system” refers to a set ofcomponents (for example, devices, and/or modules (parts)), regardless ofwhether all the components are in the same housing. Accordingly, aplurality of devices accommodated in separate housings and connectedtogether through a network, and one device obtained by accommodating aplurality of modules in one housing are both referred to as “system”.

The present embodiment can have, for example, a cloud computingconfiguration in which a plurality of devices share and cooperate toprocess one function through a network.

6. CONCLUSION

As described above, according to one embodiment of the presentdisclosure, the communication device (for example, the communicationdevice 30 or 40) determines the interleaving length of the transmit datato be transmitted through the free-space optical communication, andinterleaves the transmit data based on the determined interleavinglength. The communication device shapes the interleaved transmit data soas to make the interleaving length detectable on the receiving side ofthe free-space optical communication.

With this configuration, the communication device of the presentembodiment can change the interleaving length to any length, andtherefore, can achieve the stable communication by changing theinterleaving length according to the characteristics of the transmissionspace.

Since the communication device of the present embodiment uses theReed-Solomon product code in the forward error correction system,relatively simple implementation can ensure sufficient communicationstability against the burst errors caused by the atmosphericdisturbances.

Arranging the basic blocks makes the interleaving length easilychangeable, and allows the receiving side to detect and determine theinterleaving length by determining the SYNC codes, and thus to decodethe received data. Thereby, the interleaving length can be very easilychanged to any length according to the characteristics of thetransmission space.

The communication device of the present embodiment supports avariable-length frame, and ensures compatibility with the Ethernetphysical layer, and therefore, serves as the Ethernet media converter.As a result, the currently widely used higher-level Ethernet layers canbe used as they are. Thus, resources and risks involved in introductionof the communication system can be greatly reduced.

Since the communication device of the present embodiment can superimposethe sub-data used between devices in addition to the payload data,various types of the control information can be exchanged between theoptical communication devices. As a result, for example, theinterleaving length, the transmission output, and the transmitted lightaxis can be optimized in real time, and the stable free-space opticalcommunication can be maintained.

While the embodiment of the present disclosure has been described above,the technical scope of the present disclosure is not limited to theabove-described embodiment as it is, and can be variously changed withina scope not deviating from the gist of the present disclosure. Thecomponents ranging over different embodiments and modifications may becombined as appropriate.

The effects in the embodiment described herein are merely examples, andare not limited thereto. Other effects may be provided.

The present technology can also have the following configurations.

(1)

A communication device comprising:

an interleaving unit configured to determine an interleaving length oftransmit data to be transmitted through free-space opticalcommunication, and interleave the transmit data based on the determinedinterleaving length; and a shaping unit configured to shape theinterleaved transmit data so as to make the interleaving lengthdetectable on a receiving side of the free-space optical communication.

(2)

The communication device according to (1), wherein the shaping unit isconfigured to shape the interleaved transmit data based on a data formatconfigured to make the interleaving length detectable on the receivingside of the free-space optical communication.

(3)

The communication device according to (2), wherein

the data format is such that the transmit data includes synchronizationcodes, and

the shaping unit is configured to dispose at least two types of thesynchronization codes in the transmit data so as to make theinterleaving length detectable on the receiving side of the free-spaceoptical communication.

(4)

The communication device according to (3), wherein the shaping unit isconfigured to dispose either of a first synchronization coderepresenting a start point or an end point of the interleaving and asecond synchronization code representing a continuation point of theinterleaving at intervals of a predetermined data length based on thedetermined interleaving length.

(5)

The communication device according to (4), wherein

the data format is a block format including one or a plurality of datablocks,

the block format includes the data block or blocks each with asynchronization code disposed at a block boundary thereof, and

the shaping unit is configured to dispose either of the firstsynchronization code and the second synchronization code at the blockboundary based on the determined interleaving length.

(6)

The communication device according to (5), wherein the block format is aformat including an error correction block encoded using a block code.

(7)

The communication device according to (6), wherein the block format is aformat including an error correction block encoded using a product code.

(8)

The communication device according to (7), wherein the block format is aformat including an error correction block encoded using a Reed-Solomonproduct code.

(9)

The communication device according to any one of (1) to (8), wherein theinterleaving unit is configured to determine the interleaving lengthbased on information on a transmission path of the free-space opticalcommunication used for transmitting the transmit data.

(10)

The communication device according to (9), wherein the interleaving unitis configured to determine the interleaving length based on informationindicating at least whether the free-space optical communication iscommunication performed between a ground station and a satellitestation, or communication performed between the satellite stations.

The communication device according to (9), wherein the interleaving unitis configured to determine the interleaving length based on informationon an error rate during the communication with the receiving side of thefree-space optical communication.

(12)

The communication device according to any one of (1) to (11), wherein

the transmit data includes, in addition to first data serving as payloaddata, second data used for exchanging control information betweencommunication devices for the free-space optical communication, and

the interleaving unit is configured to determine the interleaving lengthbased on the second data.

(13)

A communication device comprising:

a detection unit configured to detect an interleaving length of receiveddata that has been received through free-space optical communication andhas been shaped such that the interleaving length is detectabletherefrom; and

a deinterleaving unit configured to deinterleave the received data basedon the detected interleaving length.

(14)

A communication method comprising:

determining an interleaving length of transmit data to be transmittedthrough free-space optical communication;

interleaving the transmit data at the determined interleaving length;and

shaping the interleaved transmit data so as to make the interleavinglength detectable on a receiving side of the free-space opticalcommunication.

(15)

A communication program for causing a computer to function as:

an interleaving unit that determines an interleaving length of transmitdata to be transmitted through free-space optical communication, andinterleaves the transmit data at the determined interleaving length; and

a shaping unit that shapes the interleaved transmit data so as to makethe interleaving length detectable on a receiving side of the free-spaceoptical communication.

Although the invention has been described with respect to specificembodiments for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art that fairly fall within the basic teaching herein setforth.

What is claimed is:
 1. A communication device comprising: circuitryconfigured to: based on information on a transmission characteristic ofa free-space optical communication used for transmitting data, determinean interleaving length of the data; interleave the data based on thedetermined interleaving length; and transmit the interleaved data,wherein the interleaving length is changeable.
 2. The communicationdevice according to claim 1, wherein the circuitry is configured toshape the interleaved data based on a data format configured to make theinterleaving length detectable on a receiving side of the free-spaceoptical communication.
 3. The communication device according to claim 2,wherein the data format is such that the data includes synchronizationcodes, and the circuitry is configured to dispose at least two types ofthe synchronization codes in the data so as to make the interleavinglength detectable on the receiving side of the free-space opticalcommunication.
 4. The communication device according to claim 3, whereinthe at least two types of the synchronization codes includes: a firstsynchronization code representing a start point or an end point of theinterleaving, and a second synchronization code representing acontinuation point of the interleaving at intervals of a predetermineddata length based on the determined interleaving length.
 5. Thecommunication device according to claim 4, wherein the data format is ablock format including one or a plurality of data blocks, the blockformat includes the data block or blocks each with a synchronizationcode disposed at a block boundary thereof, and the circuitry isconfigured to dispose either of the first synchronization code and thesecond synchronization code at the block boundary based on thedetermined interleaving length.
 6. The communication device according toclaim 5, wherein the block format is a format including an errorcorrection block encoded using a block code, a product code or aReed-Solomon product code.
 7. The communication device according toclaim 1, wherein the information on a transmission characteristicfurther comprises information on an error rate of the free-space opticalcommunication.
 8. The communication device according to claim 1, whereinthe data includes, in addition to first data serving as payload data,second data used for exchanging control information betweencommunication devices for the free-space optical communication, and thecircuitry is configured to determine the interleaving length based onthe second data.
 9. The communication device according to claim 1,wherein the information on a transmission characteristic comprisesinformation indicating that the free-space optical communication is:communication performed between a ground station and a satellitestation, or communication performed between satellite stations, orcommunication performed between ground stations.
 10. The communicationdevice according to claim 9, wherein: based on the information on thetransmission characteristic indicating that the free-space opticalcommunication is the communication performed between the satellitestations, the interleaving length is determined to be a firstinterleaving length; based on the information on the transmissioncharacteristic indicating that the free-space optical communication isthe communication performed between the ground station and the satellitestation, the interleaving length is determined to be a secondinterleaving length longer than the first interleaving length; and basedon the information on the transmission characteristic indicating thatthe free-space optical communication is the communication performedbetween the ground stations, the interleaving length is determined to bea third interleaving length longer than the first interleaving lengthand shorter than the second interleaving length.
 11. A communicationdevice comprising: circuitry configured to received data through afree-space optical communication; detect an interleaving length of thereceived data; and deinterleave the received data based on the detectedinterleaving length, wherein the interleaving length is based oninformation on a transmission characteristic of the free-space opticalcommunication used for transmitting the data, wherein the interleavinglength is changeable.
 12. The communication device according to claim11, wherein the information on a transmission characteristic furthercomprises information on an error rate of the free-space opticalcommunication.
 13. The communication device according to claim 11,wherein the information on a transmission characteristic comprisesinformation indicating that the free-space optical communication is:communication performed between a ground station and a satellitestation, or communication performed between the satellite stations, orcommunication performed between ground stations.
 14. The communicationdevice according to claim 13, wherein: based on the information on thetransmission characteristic indicating that the free-space opticalcommunication is the communication performed between the satellitestations, the interleaving length is determined to be a firstinterleaving length; based on the information on the transmissioncharacteristic indicating that the free-space optical communication isthe communication performed between the ground station and the satellitestation, the interleaving length is determined to be a secondinterleaving length longer than the first interleaving length; and basedon the information on the transmission characteristic indicating thatthe free-space optical communication is the communication performedbetween the ground stations, the interleaving length is determined to bea third interleaving length (longer than the first interleaving lengthand shorter than the second interleaving length.
 15. A communicationmethod performed by a communication device including a processor, themethod comprising: based on information on a transmission characteristicof a free-space optical communication used for transmitting data,determining an interleaving length of the data; interleaving the databased on the determined interleaving length; and transmitting theinterleaved data, wherein the interleaving length is changeable.
 16. Themethod according to claim 15, wherein the information on a transmissioncharacteristic further comprises information on an error rate of thefree-space optical communication.
 17. The method according to claim 15,wherein the information on a transmission characteristic comprisesinformation indicating that the free-space optical communication is:communication performed between a ground station and a satellitestation, or communication performed between satellite stations, orcommunication performed between ground stations.
 18. The methodaccording to claim 17, wherein: based on the information on thetransmission characteristic indicating that the free-space opticalcommunication is the communication performed between the satellitestations, the interleaving length is determined to be a firstinterleaving length; based on the information on the transmissioncharacteristic indicating that the free-space optical communication isthe communication performed between the ground station and the satellitestation, the interleaving length is determined to be a secondinterleaving length longer than the first interleaving length; and basedon the information on the transmission characteristic indicating thatthe free-space optical communication is the communication performedbetween the ground stations, the interleaving length is determined to bea third interleaving length (longer than the first interleaving lengthand shorter than the second interleaving length.